We used ProZinc for 3 months before switching to Lantus. IMHO, ProZinc is an excellent insulin, and so is Lantus. I have no experience with BCP PZI. I have heard good and bad things about every insulin in use. It really isn't about what is the best insulin, it's about what is the best insulin for your cat and your situation.
If you go to the PZI forum, there is a ongoing discussion about cost that you might find of interest.
If you can't stick to a 12/12 schedule, I would switch to a PZI. Lantus really requires a strict 12/12 schedule. The nice thing about the PZIs is the flexibility, both in schedule and dosing.